Method of inoculating nodular cast iron

ABSTRACT

An improved method of inoculating nodular cast iron to obtain uniform and thorough inoculation of nodular iron simultaneously with the casting thereof is disclosed. A preformed inoculant agent body having perforations extending between the opposing faces thereof and congruent ferrous metal cover plates contacting the body faces is placed in a stationary position across the gating system of the mold so as to cause the molten iron entering the mold to flow through the perforations uniformly dissolving the inoculant agent on contact and being inoculated thereby just prior to casting thereof. The ferrous metal cover plates extend the life of the inoculant body and result in more uniform inoculation and, consequently, a more uniform microstructure in the cast iron than otherwise obtainable.

[451 Apr. 25, 1972 [54] METHOD OF INOCULATING NODULAR CAST IRON [72] Inventors: Edward F. Ryntz, Jr., Warren; Thomas E.

OConner, Mount Clemens, both of Mich.

[73] Assignee: General Motors Corporation, Detroit,

Mich.

[22] Filed: Nov. 30, 1970 [2]] Appl. No.: 93,632

1,132,055 10/1968 GreatBritain.....

Great Britain .l: 164/55 Primary Examiner-4. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Sidney Carter and Peter P. Kozak 1 ABSTRACT An improved method of inoculating nodular cast iron to obtain uniform and thorough inoculation of nodular iron simultaneously with the casting thereof is disclosed. A preformed inoculant agent body having perforations extending between the opposing faces thereof and congruent ferrous metal cover plates contacting the body faces is placed in a stationary position across the gating system of the mold so as to cause the molten iron entering the mold to flow through the perforations uniformly dissolving the inoculant agent on contact and being inoculated thereby just prior to casting thereof. The ferrous metal cover plates extend the life of the inoculant body and result in more uniform inoculation and, consequently, a more uniform microstructure in the cast iron than otherwise obtainable.

4 Claims, 3 Drawing Figures PATENTEDAPRZS I972 3,658,115

INV EN TORS METHOD OF INOCULATING NODULAR CAST IRON This invention relates to the production of nodular cast iron and, more particularly, to an improved method of inoculating nodular iron to obtain uniform and thorough inoculation of 5 the nodular iron simultaneously with the casting thereof.

In current production practice, relatively soft base gray iron is rendered nodular by a small addition of magnesium which changes the shape of the flake graphite to a nodular or spheroidal form. However, magnesium also strongly promotes the formation of cementite (Fe C) which is detrimental to the impact strength and ductility of nodular iron castings. If the nodular iron is to have good ductility without subsequent costly heat treatments, a silicon inoculation must be made to the melt after the magnesium treatment to eliminate this carbide-forming tendency in the treated iron. Alloys typically used for inoculation include the ferrosilicons containing either 75 or 85 percent silicon, calcium-bearing ferrosilicons with 85 percent silicon, or various combinations of these alloys. The use of ferrosilicon inoculants has the advantages of eliminating carbides in small castings or thin sections in the as-cast components, of increasing the ferrite content of the as-cast component, and of increasing the graphite nodule count and attaining a more uniform nodule size to obtain improved mechanical properties in addition to those resulting from elimination of carbides and from increased ferrite counts.

However, in post inoculation treatments a phenomenon known as fade of the post-inoculation occurs rapidly decreasing the effectiveness of the inoculant addition as the metal is held before casting. To overcome the loss of inoculation effect with time various techniques have been devised for adding the inoculating agent as late in the pouring process as possible. These techniques include mold inoculation, instantaneous ladle inoculation and the use of inoculant mold inserts.

Mold inoculation involves placing in the gating system of the mold a small quantity, usually on the order of a few grams, of ferrosilicon fines which are effective to inoculate the molten iron as the casting is poured. The disadvantage of this method is that the initial metal entering the mold dissolves most of the ferrosilicon fines and is effectively treated, but the subsequent metal receives little treatment which results in undesirable microstructural variations throughout the casting. A more consistent microstructure can be obtained by instantaneous ladle inoculation wherein a solid ferrosilicon rod positioned against the lip of the pour ladle is dissolved by the stream of molten metal as it flows from the ladle into the mold. Dissolution of the inoculant thereby occurs throughout the casting process eliminating the variation which occurs with the use of ferrosilicon fines in the mold. A further improvement in post-inoculation effectiveness has been found in the use of preformed inoculant mold inserts. lnoculant mold inserts are preformed, perforated cores of ferrosilicon particles bonded with sodium silicate or similar binder to form a hardened insert of the inoculant agent. The hardened inoculant insert is placed in a desired position in the gating system of the mold whereby the iron on casting flows through the perforations or holes in the insert dissolving the ferrosilicon particles on contact. This late inoculation just prior to solidification of the casting results in the desired microstructures. The use of stationary mold inserts is described in British Pat. No. 1,132,055 and a variation thereof is described in British Pat. No. 1,132,056, both issued on Oct. 30, I968.

The principal problem encountered with the use of ferrosilicon mold inserts of the type described in the aforementioned British patents is one of too rapid dissolution or catastrophic break-up of the insert by the molten iron flowing therethrough. When this occurs, an irregular distribution of ferrosilicon particles in the melt results which produces a variation in the microstructure of the casting similar to that found when the mold inoculant with ferrosilicon fines method is employed. This problem becomes particularly acute when the quantity of metal poured is large, e.g., greater than 100 pounds, as typically occurs in production foundries, or when the pour rate is extremely slow or fast.

We have found, however, that the problem of rapid dissolution and catastrophic break-up of the preformed inoculant agent on casting can be overcome by the provision of an improved mold insert adapted to be positioned in the gating system of the mold in a stationary position and comprising a preformed inoculation agent insert and a pair of correspondingly shaped steel cover plates contacting the faces of the inoculant insert. These plates protect the inoculant agent from rapid dissolution by the molten metal flowing therethrough allowing for uniform dissolution of the insert throughout the duration of the pour which results in a uniform microstructure throughout the casting.

Other features and advantages of our invention will become more apparent from the following detailed description of our invention reference being had to the accompanying drawings, of which:

FIG. 1 is a pictorial illustration of the preferred form of our improved mold insert;

FIG. 2 is a cross-sectional view of a mold and two embodiments of our improved mold insert position therein; and

FIG. 3 is a pictorial illustration of another form of our improved mold insert.

Referring now to the drawings, the improved mold insert 10 comprises a generally disc-shaped, preformed body 12 of a nodular iron inoculant agent having a plurality of perforations or holes 14 extending therethrough between the generally parallel faces of the body. Contacting both faces of the preformed body 12 are metallic cover plates 16 and 18 having a shape corresponding to that of the preformed body and a plurality of holes 20 congruent with the holes 14 of the body 12 to provide continuous, unobstructed passages through the mold insert 10.

The inoculant agent body 12 is formed by pressing particles of inoculant agent, which may be any of the well-known agents and alloys used for inoculation of nodular cast iron, along with a suitable binder in a die to form a body of desired configuration. Typically used nodular iron inoculant agents useful in this invention are generally the ferrosilicon alloys containing up to about, by weight, 50 percent iron, 2 percent calcium, 10 percent magnesium and balance silicon. A preferred inoculant agent is a 75 percent calcium-bearing ferrosilicon. In the preferred embodiment, the agent is pressed in particle form of a size from about 20 to mesh (Tyler scale) with about 12 percent, by weight, sodium silicate binder. A suitable binder is Type 0 sodium silicate, a commercial product of the Philadelphia Quartz Co. After forming the body is cured in a furnace at a temperature of about 300 400 F for about 10 minutes or is microwave cured for several minutes to form a hardened body. Although it is not necessary that the cover plates 16 and 18 be bonded to the inoculant agent preform 12, if desired, the faces of the preform can be coated with a thin layer of sodium silicate and the cover plates bonded thereto.

After forming, the mold insert is disposed in a stationary position within the gating system of the mold. As used herein, the term gating system" includes the downsprue, runners and gate of the mold. Referring to FIG. 2 wherein there is shown a representative mold 22 including a mold cavity 24, runners 26 and a downsprue 28, it may be seen that the mold insert 10 extends across the gating system with the faces of the insert being substantially perpendicular to and the perforations l4 and 20 being substantially aligned with the direction of metal flow through the gating system at the insert such that the metal is caused to flow through congruent holes 20 and 14 and into the mold cavity 24. The insert can be either formed in the mold during the mold forming operation or placed in the mold after forming. The size and shape of the mold insert and of the through-holes can be adjusted experimentally to the rate of flow of the molten iron, the pouring time of the casting and the particular geometry of the mold gating system. For example, in some foundry operations molds are typically formed with trapazoidal-shaped runners. In this case a mold insert 30, as shown in FIG. 3, which includes a preformed inoculant agent body 31, formed as previously described, having a trapazoidalshaped body portion 30a and a base portion 30b with a plurality of holes 32 through the body portion 300 and having a pair of correspondingly shaped cover plates 34 and 36 contacting the opposed faces of the body 31 may be placed in the mold during forming of the mold with the body portion 30a extending across the runner and the base portion 30b being embedded in the mold to hold the insert 30 in place, as illustrated in FIG. 2.

The requirements for the cover plates which contact the faces of the inoculant agent body to form the mold insert are two-fold. First, since nodular iron is typically poured at a temperature between about 2,500 and 2,600 F, the plates must be formed of a material having a melting point high enough to prevent too rapid dissolution of the plates. Secondly, since the plates are gradually dissolved along with the inoculant agent body by the molten nodular iron flowing through the holes in the insert with the holes-thereby gradually increasing in size, the composition of the plates must be compatible with that of the nodular iron to prevent undue alteration of the casting chemistry. Accordingly, ferrous metal cover plates are generally required in the practice of our invention. The use of low carbon steel plates has shown good results in the practice of our invention.

Tests conducted with mold inserts of the type shown in FIG. 1, positioned in a mold as shown in FIG. 2, showed at least a two-fold increase in the life of the mold insert as opposed to the life of a similar insert without the cover plates. It was found that after casting some of the insert still remained indicating improved dissolution and distribution of the inoculating agent. The effectiveness of the mold insert in eliminating carbides and in promoting a ferritic matrix allows for decreasing the amount of post-inoculant added to the pour ladle which permits increased silicon levels in the base iron, allowing for the use of higher silicon scrap materials in thefurnace charge. in addition, furnace refractory life is extended by higher silicon levels in the melt.

Although our invention has been described in terms of certain specific embodiments, it will be recognized that other forms may be adopted within the scope of our invention. For example, if desired, only one cover plate can be used in forming the mold insert to achieve some of the benefits of our invention. If one plate is used, it may be placed either on the molten metal entrance side of the insert or on the molten metal exit side, depending upon the mold gating design and method of positioning the insert in the mold.

We claim:

1. A method of inoculating molten nodular cast iron simultaneously with the casting thereof comprising,

providing a mold including a mold cavity and a gating system communicating with said mold cavity, disposing within said mold an inoculant mold insert comprising a preformed body of a nodular iron inoculant agent, said body having generally parallel opposing faces with a plurality of perforations extending through said body between said faces, and at least one correspondingly shaped, congruently perforated ferrous metal cover plate contacting one of said faces of said body, said insert extending across said gating system with said faces being substantially perpendicular. to and said perforations being substantially aligned with the direction of flow of said molten iron through said gating system at said insert,

introducing said molten iron into said gating system, and

causing said molten iron to flow through said gating system,

through said perforations in said insert uniformly dissolving the inoculant-agent on contact and being inoculated thereby while flowing therethrough, and into said mold cavity to form a casting therein.

2. A method of inoculating molten nodular cast iron simultaneously with the casting thereof comprising,

providing a mold including a mold cavity and a gating system communicating with said mold cavity, disposing within said mold an inoculant mold insert comprising a reformed body of a nodular iron inoculant agent, sai body having generally parallel opposing faces with a plurality of perforations extending through said body between said faces, and a correspondingly shaped, congruently perforated ferrous metal cover plate contacting each of said faces of said body, said insert extending across said gating system with said faces being substantially perpendicular to and said perforations being substantially aligned with the direction of flow of said molten iron through said gating system at said insert,

introducing said molten iron into said gating system, and

causing said molten iron to flow through said gating system,

through said perforations in said insert uniformly dissolving the inoculant agent on contact and being inoculated thereby while flowing therethrough, and into said mold cavity to form a casting therein.

3. A method of inoculating molten nodular cast iron with a ferrosilicon inoculant agent simultaneously with the casting thereof comprising,

providing a mold including a mold cavity and a gating system communicating with said mold cavity,

disposing within said mold an inoculant mold insert comprising a preformed body of bonded particulate ferrosilicon inoculant agent, said body having generally parallel opposing faces with a plurality of perforations extending through said body between said faces, and at least one correspondingly shaped, congruently perforated ferrous metal cover plate contacting one of said faces of said body, said insert extending across said gating system with said faces being substantially perpendicular to and said perforations being substantially aligned with the direction of flow of said molten iron through said gating system at said insert,

introducing said molten iron into said gating system, and

causing said molten iron to flow through said gating system,

through said perforations in said insert uniformly dissolving the ferrosilicon particles on contact and being inoculated thereby while flowing therethrough, and into said mold cavity to form a casting therein.

4. A method of inoculating molten nodular cast iron simul taneously with the casting thereof comprising,

providing a mold including a mold cavity and Ya gating system communicating with said mold cavity,

disposing within said mold an inoculant mold insert comprising a preformed body of a nodular iron inoculant agent, said preformed body having a base portion and a body portion with said body portion having generally parallel opposing faces with a plurality of perforations extending through said body portion between said faces of said body portion, and at least one correspondingly shaped, congruently perforated ferrous metal cover plate contacting one of said faces of said body portion, said base portion of said insert being embedded in said mold and said body portion of said insert extending across said gating system with said faces of said body portion being substantially perpendicular to and said perforations being substantially aligned with the direction of flow of said molten iron through said gating system at said insert, introducing said molten iron into said gating system, and causing said molten iron to flow through said gating system,

through said perforations in said insert uniformly dissolving the inoculant agent on contact and being inoculated thereby while flowing therethrough, and into said mold cavity to form a casting therein.

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2. A method of inoculating molten nodular cast iron simultaneously with the casting thereof comprising, providing a mold including a mold cavity and a gating system communicating with said mold cavity, disposing within said mold an inoculant mold insert comprising a preformed body of a nodular iron inoculant agent, said body having generally parallel opposing faces with a plurality of perforations extending through said body between said faces, and a correspondingly shaped, congruently perforated ferrous metal cover plate contacting each of said faces of said body, said insert extending across said gating system with said faces being substantially perpendicular to and said perforations being substantially aligned with the direction of flow of said molten iron through said gating system at said insert, introducing said molten iron into said gating system, and causing said molten iron to flow through said gating system, through said perforations in said insert uniformly dissolving the inoculant agent on contact and being inoculated thereby while flowing therethrough, and into said mold cavity to form a casting therein.
 3. A method of inoculating molten nodular cast iron with a ferrosilicon inoculant agent simultaneously with the casting thereof comprising, providing a mold including a mold cavity and a gating system communicating with said mold cavity, disposing within said mold an inoculant mold insert comprising a preformed body of bonded particulate ferrosilicon inoculant agent, said body having generally parallel opposing faces with a plurality of perforations extending through said body between said faces, and at least one correspondingly shaped, congruently perforated ferrous metal cover plate contacting one of said faces of said body, said insert extending across said gating system with said faces being substantially perpendicular to and said perforations being substantially aligned with the direction of flow of said molten iron through said gating system at said insert, introducing said molten iron into said gating system, aNd causing said molten iron to flow through said gating system, through said perforations in said insert uniformly dissolving the ferrosilicon particles on contact and being inoculated thereby while flowing therethrough, and into said mold cavity to form a casting therein.
 4. A method of inoculating molten nodular cast iron simultaneously with the casting thereof comprising, providing a mold including a mold cavity and a gating system communicating with said mold cavity, disposing within said mold an inoculant mold insert comprising a preformed body of a nodular iron inoculant agent, said preformed body having a base portion and a body portion with said body portion having generally parallel opposing faces with a plurality of perforations extending through said body portion between said faces of said body portion, and at least one correspondingly shaped, congruently perforated ferrous metal cover plate contacting one of said faces of said body portion, said base portion of said insert being embedded in said mold and said body portion of said insert extending across said gating system with said faces of said body portion being substantially perpendicular to and said perforations being substantially aligned with the direction of flow of said molten iron through said gating system at said insert, introducing said molten iron into said gating system, and causing said molten iron to flow through said gating system, through said perforations in said insert uniformly dissolving the inoculant agent on contact and being inoculated thereby while flowing therethrough, and into said mold cavity to form a casting therein. 